Copolymers of ethylene and a vinyl ester of an aliphatic carboxylic acid, such as vinyl acetate, etc., have been used as a property improving agent for rubbers and synthetic resins because of their excellent properties. In particular, such copolymers have been noted with keen interest as blending agents for improving the properties such as processability, impact strength, melt fluidity, etc., of vinyl chloride resins.
A suitable form of such blending agents for accomplishing uniform mixing is preferably a powdery form. However, VEE resins are generally produced by suspension polymerization or by emulsion polymerization method, and the copolymer obtained as a single sized form by such polymerization methods is in a state wetted by the aqueous medium. Accordingly, it is necessary to remove the aqueous medium by drying. However, on drying, the copolymer particles tend to agglomerate with each other owing to the low softening point and high tackiness of the copolymer, and hence it is very difficult to obtain the VEE resin in a dry powdery form. This tendency is particularly severe in VEE resins having a vinyl ester content of about 40 to 70%, suitable for use as blending agents for PVC.
Various approaches have been proposed for forming VEE resin powders while preventing them from agglomerating. For example covering a powdery VEE resin with a fine powder of an inorganic compound or an organic compound is proposed in U.S. Pat. No. 3,463,751. However, in this case, the fine powder attached to the surfaces of the VEE resin particles is liable to be embeded in the VEE particles with the particles reverting to rubbery resin particles during storage. In particular, under loaded conditions, the anti-sticking capability will be lost causing agglomeration of the particles. On the other hand, if a large amount of the fine powder is used, a part of the fine powder falls off from the particles and accumulates on the bottom of the container, and a homogeneous VEE resin is not obtained. There are other disadvantages in the case of blending a resin such as PVC, etc., with such VEE resin particles. When molding or forming the resin, the transparency of the resin is lowered and when using the VEE particles as a solution, the solution becomes opaque or forms precipitates.
British Pat. No. 1,511,146 describes that sticking of VEE resin particles may be prevented by coating the surface thereof with a surface active agent. However, although coating with a surface active agent may give an anti-sticking effect when the content of vinyl ester in the VEE resin is low, that effect becomes inadequate when the vinyl ester content is increased, and the resin particles are liable to agglomerate during storage. Increasing the amount of surface active agent causes the resin particles to be become sticky due to the presence of the surface active agent, and the water absorption resistance of the resin is lowered owing to the hydrophilic property of the surface active agent.